This invention relates to the fabrication of polarization retaining single-mode (PRSM) optical fibers and more particularly to the fabrication of preforms from which fibers having elliptically-shaped cores can be drawn.
In many applications of single-mode optical fibers, eg. gyroscopes, sensors and the like, it is important that the propagating optical signal retain the polarization characteristics of the input light in the presence of external depolarizing perturbations. This requires the waveguide to have an azimuthal asymmetry of the refractive index profile.
One of the first techniques employed for improving the polarization performance of single-mode fibers was to distort the symmetry of the core. A method of making this kind of PRSM fiber is disclosed in U.S. Pat. No. 5,149,349, which is incorporated herein by reference. A PRSM optical fiber is formed by drawing a fiber from a draw blank having a glass core surrounded by cladding glass containing apertures that are diametrically opposed with respect to the core. The fiber is drawn at such a rate and temperature that the apertures close and the core becomes elliptically-shaped. In a preferred method of making the draw blank, longitudinal grooves are formed on diametrically opposed sides of a cylindrically-shaped core preform in which the glass core is surrounded by the cladding glass. Glass particles are deposited on the outer surface of a glass tube, and the core preform is inserted into the tube. The resultant assembly is heated to sinter or consolidate the particles, whereby the tube is collapsed and fused to the grooved core preform to form an assembly having longitudinal apertures on opposite sides of the core.
When the aperture-containing blank is drawn to form the PRSM fiber, the apertures close due to surface tension and the flow of molten glass into the apertures. This flow causes the round core of the blank to become elongated in the direction of the apertures. The core aspect ratio of the elliptically-shaped core is primarily determined by the spacing between the core and the apertures. As the spacing between the core and the apertures in the draw blank decreases, the core of the resultant fiber becomes more elongated in cross-section, but the core cross-section tends to have long thin ends. If the spacing is too small, the core may break through the cladding region between core and apertures, resulting in the formation of a core the cross-section of which is flattened with undesirable flared ends. By xe2x80x9cendsxe2x80x9d is meant the tips of the elongated core along the major axis of the core as observed in a plane perpendicular to the fiber longitudinal axis.
It is therefore an object of the invention to provide a method of making PRSM optical fibers which overcomes the disadvantages of the prior art. A further object is to provide a PRSM fiber producing method which is relatively simple to practice. Yet another object is to provide a method for making elliptical core PRSM fibers having improved core cross-sectional shape.
In accordance with the present method, a polarization maintaining single-mode optical fiber is formed by initially forming an optical fiber draw blank having a glass core region of refractive index n1 surrounded by a cladding glass region of refractive index n2. The cladding region includes apertures that are diametrically opposed with respect to the core region and that are spaced from the core region. The draw blank has a region of low viscosity glass between the core region and the apertures, the low viscosity glass region having a refractive index n3 and a viscosity lower than that of the cladding glass region. An optical fiber is drawn from the draw blank at such a rate that the apertures close and the core becomes elliptical in cross-section.
Another aspect of the invention pertains to an optical fiber having an elliptically-shaped core of aspect ratio xcfx811, where xcfx811 equals b1/a1, b1 being the major axis radius and a1 being the minor axis radius of the core. Surrounding the core is a low viscosity glass region of elliptically-shaped cross-sectional configuration having an aspect ratio xcfx812 that is equal to b2/a2, b2 being the major axis radius and a2 being the minor axis radius of the low viscosity region. The aspect ratio xcfx812 of the low viscosity region is less than xcfx811. A cladding glass region surrounds the low viscosity region. The viscosity of low viscosity region is lower than that of the cladding glass region.